Global Warming Is Easy to Understand
We have all heard, by now, that ninety-some-odd percent of scientists believe that climate change is real and that human beings are causing it. Nevertheless, we continue to hear the so-called debate about it rage on, and we see polls such as this one that indicate that approximately 70% of Americans believe climate change is real, but that also show a stark partisan divide when it comes to who believes what. What is going on with those other 30%? Why are they so resistant to believe that climate change is happening?
I believe a major component of the reason is that nobody wants to be told what to think by people they don’t know, especially when obvious political and economic agendas are aligned on both sides of this issue. While it is the case that climate change is real, it is also true that it is a fallacious argument to say “it’s real because scientists say so.” That is an appeal to authority and not a convincing argument. People are right to be skeptical when faced with such arguments. It is doubly irresponsible on the part of people trying to educate the public, in my view, when one considers that climate change is not rocket science, and doesn’t require an advanced degree to understand when it comes to the basics. On an intuitive level, climate change is easy to understand.
The Greenhouse Effect
The greenhouse effect is the driving mechanism behind climate change, but what is it? On a fundamental level, it stipulates that a planetary atmosphere can retain heat, and that certain gases retain more heat than others. Gases that promote a particularly high level of heat retention are called greenhouse gases. To illustrate this, we can look to our nearest neighbors in space: Mercury, Venus, and Mars.
Mercury is the closest planet to the Sun, less than half the distance between Earth and the Sun. Mercury, like our own Moon, has no atmosphere, and thus no ability to have any greenhouse effect. What this means is that Mercury retains less warmth from the Sun, and seeing as how Mercury only rotates three times on its axis for every two orbits of the Sun it completes, it has a dark side and a light side. On the dark side of Mercury, the surface temperature bottoms out at a chilly -315 °F. On the light side, facing the Sun, it reaches a scorching 800 degrees at its equator. The poles never rise above -136 °F. So, despite being the closest planet to the Sun, Mercury has colder poles than Earth, and a much colder night. This is because Earth has an atmosphere that retains some fraction of the energy that it receives from the Sun. Wind and ocean currents distribute that heat around the globe, preventing the poles from being as cold as the poles on Mercury. In fact, the lowest temperature ever recorded on Earth was on Antartica, near the South Pole, and was -128.6 °F, which is still warmer than the maximum temperature at Mercury’s poles.
Venus is the next planet in orbit around the Sun at a little less than 3/4 the distance from the Sun as Earth. Venus has a thick atmosphere, that comprises over 95% carbon dioxide, with the bulk of the remainder being nitrogen and sulfur dioxide. We know that carbon dioxide is supposed to be a greenhouse gas, so for a planet like Venus where most of the atmosphere is carbon dioxide, what does that mean? It means that Venus is hot. Very hot. The surface of Venus is a sweltering 863 °F. Also, despite Venus, like Mercury, having an exceptionally slow rate of axial rotation, with one side staying dark for long periods (1 Venutian day is about 225 Earth days), Venus has almost no variation in temperature between night and day. This is because little heat escapes a thick atmosphere that is almost entirely carbon dioxide, and Venus’s atmosphere is definitely thick, as it has about 93 times the mass of Earth’s atmosphere.
Our last nearby neighbor is Mars, which orbits the Sun at about 1.5 the distance that Earth does. Mars is smaller than Earth or Venus, but it is still large enough to retain a thin atmosphere. Very thin. The mean atmospheric pressure on the surface of Mars is only about 0.6% that on the the surface of Earth. This is less than 1/5 the atmospheric pressure at the peak of Mt. Everest. One might think an atmosphere this meager would be insufficient to retain very much heat, but Mars’s atmosphere is also largely carbon dioxide; it is 96% carbon dioxide, 1.93% argon, 1.89% nitrogen, with trace amounts of other gases. This results in temperature ranges on the surface of Mars from -225 °F at the poles in the winter (Mars has seasons like Earth) to 95 °F at the equator in the summer. Despite receiving only about 6.5% as much energy from the Sun that Mercury does, Mars still has higher temperatures on its side facing away from the Sun. Just like on Earth, the Sun will not rise at the poles during winter for months at a time yet, despite receiving no direct sunlight for all that time, the Martian poles stay warmer than the dark side of Mercury. This would be impossible without an atmosphere that can retain heat.
Looking at our neighbors only gives us the proof of concept of a greenhouse effect; it does not explain how it works. We know that planetary atmospheres retain heat, and we know that Venus’s thick atmosphere of carbon dioxide does a particularly good job of it, but why? It turns out to be a simple consequence of quantum mechanics, of the way light is absorbed and emitted by atoms and molecules.
When a photon of light hits a molecule, there are several possible outcomes. It may bounce off and be reflected or scatter in another direction, it may pass right through, or it may get absorbed, adding its energy to that of the molecule in question. The first case is what happens to visible light in the atmosphere, and it is why the sky is blue. Blue light that hits the molecules in the atmosphere scatters around in all directions, and some of those photons end up in our eyes, making the sky look blue. The second case is the basis for such ideas as “X-ray vision,” which allows superheroes to look through walls. It also happens to be what happens at the doctor’s office when you get an X-ray; the X-rays are able to pass through the skin and other tissue and let the doctor see the state of our bones. The third, absorption, is what makes the sand on the beach get hot in the summer Sun. The light from the Sun hits it all day, warming it up, leaving it painful to walk on with bare feet in the afternoon.
We know from getting X-rays performed at the doctor’s office that they do not penetrate lead. We are instructed to wear pads containing lead to block them from getting into areas of the body that don’t need to be investigated, becasue X-rays do damage to our cells. X-rays and visible light, along with infrared, ultraviolet, microwaves, radio waves, and gamma rays, make up the electromagnetic spectrum. Each range corresponds to a different set of wavelengths, and interacts with different types of molecules and materials differently. Visible light passes right through the air (getting scattered along the way), which makes it possible for us to see. However, it clearly does not penetrate the walls of our room, because we can’t see through them. Ultraviolet light gives us sunburn, but if we wear hats, clothes, and sunscreen, they are blocked out. It happens that certain molecules, such as carbon dioxide, are able to absorb infrared radiation, which is the wavelength of light that carries heat. Other greenhouse gases are the same way; they can interact with infrared light by absorbing it. Nitrogen, the primary constituent of our atmosphere, is not a greenhouse gas, meaning that infrared light passes right through or scatters around it.
We know first-hand that the Earth absorbs light and warms up. We have all walked on a beach or a sidewalk or through a parking lot that has been basking in the heat of the day; some of us have learned painful lessons this way. We can think of this like a pizza sitting in an oven, absorbing heat. When we take the pizza out of the oven and set it on the counter, it cools. It emits the excess energy it absorbed and will eventually settle to the temperature of the room around it. So, too, with the Earth at night, which is why sidewalks and the sand on the beach cool down overnight, and why average temperatures at night across the globe are lower than average temperatures during the day. The process by which this happens is infrared radiation. If you looked at a hot pizza through infrared goggles, you would see it glowing. This technology also lets people spot each other at night, since the human body itself radiates heat, and has been used to equip soldiers for nighttime operations. In fact, anything hot that is cooling down is emitting infrared radiation, or giving off heat and, since we know that the Earth itself cools down overnight when it isn’t receiving energy from the Sun, we know that it is emitting infrared light.
This is where greenhouse gases come into play, and where quantum mechanics dictates why they cause an atmosphere to retain heat. When an infrared photon hits a molecule of a greenhouse gas, it can be absorbed by that molecule. It will later be reemitted, in a random direction. This is the important part: the incident infrared photon and the infrared photon that is emitted later are not necessarily emitted in the same direction. Thus, if you picture an infrared photon being emitted upwards from the surface of the Earth and hitting a molecule of carbon dioxide in the air, that molecule may emit it back down at Earth, or higher up into the atmosphere. If it goes back to the Earth, it will be absorbed and reemitted later again. If it is emitted out to space, it is lost forever, and it represents a reduction in the total energy content of Earth. Every night, this process happens countless times, with infrared photons being exchanged between the surface of the Earth and the atmosphere, with a net loss that causes the surface of the Earth to cool. We learned before, however, that a thicker atmosphere of greenhouse gases, such as that present on Venus, shows almost no net loss of energy between night and day. While an infrared photon emitted from Earth may bounce back and forth a few times, or get exchanged between a series of carbon dioxide molecules in the atmosphere, it will eventually make its way out into space. On Venus, it stays trapped in the atmosphere forever, bouncing between the planetary surface and back and forth between the molecules in the atmosphere, never making it back out into space.
When you understand this mechanism of action, of how heat is exchanged and transported, it is obvious that the more greenhouse gas that is present, the more heat will be retained. But, another argument of climate change deniers is that the Earth is huge and there is no way humankind could make a measurable difference in terms of the quantity of carbon dioxide present in the atmosphere. If that is true, then it is no cause for concern. Perhaps the carbon dioxide we put out is so insignificant in the face of the carbon dioxide already present that it makes no difference, or perhaps the Earth reabsorbs carbon into the ocean and forests faster than we can emit it into the atmosphere, and that we are not able to push carbon dioxide out of equilibrium with our meager production. We only need to look at the numbers to discern the truth.
The Human Effect
The total mass of the Earth’s atmosphere is 5.15 x 10¹⁸ kg. To put that into perspective, the total mass of all people in the world is about 4.9 x 10¹² kg, meaning the Earth’s atmosphere has about a million times the mass of the entire population of human beings. We are small compared to the atmosphere, but not astronomically small. The percentage of the Earth’s atmosphere by mass of carbon dioxide is 0.0582%, or about 3 x 10¹⁵ kg. How does this compare to the amount of carbon dioxide we are adding to the atmosphere every year?
The EPA maintains extensive records of greenhouse gas emissions going back for years. We see from this data that the carbon dioxide emissions in the United States have been at least 5,000 MMT (millions of metric tonnes) every year they have been keeping track. A metric tonne is 1,000 kg, so this amounts to at least 5 x 10¹² kg of carbon dioxide emissions per year in the United States alone, and the United States is no longer the world’s leading emitter of carbon dioxide. The United States accounts for a little less than 20% of the global carbon dioxide emissions which, in 2013, were approximately 3.6 x 10¹³ kg. This is about 0.12% of the total carbon dioxide in the atmosphere. This may seem small, but if we maintain this rate for 10 years, then we will have increased the total amount of carbon dioxide by 1.2%; if we keep it up for 20 years, that rises to 2.4%. The alarming part, of course, is that the EPA records show that we already have been emitting carbon dioxide at potentially deleterious levels for decades; we have already added measurably to the amount of carbon dioxide in the atmosphere.
These are small numbers, but these are not negligibly small numbers. Every little bit of extra carbon dioxide in the atmosphere means that a patch of the surface of the Earth that rotates away from the Sun at night will be a little warmer than it would have been without the excess carbon by the time it rotates back around to face the Sun in the morning and, as such, it will be even warmer in the heat of the day when it has absorbed more energy, because it will have started at a higher point in the morning than it otherwise would have. In other words, as we add more carbon dioxide to the atmosphere, we reset the temperature equilibrium on Earth, thereby driving up the average global temperature.
While an increase of carbon dioxide in the atmosphere of 1.2% over 10 years or 2.4% over 20 years may seem small, its effects are not, because other mechanisms in place on Earth compound this and cause global warming to accelerate. The chief culprit in this is water, which covers most of the Earth’s surface, because of how water interacts with light in its various forms that are present on Earth.
As more energy is retained by the Earth’s atmosphere, the global average temperature goes up. Even a slight rise in average temperatures means that the presence of ice all around the world diminishes by some amount. The polar caps shrink and the snowpack in mountainous regions disappears. This is relevant because ice has a high albedo, which is a measurement of the reflectivity of a substance, generally in reference to the visible light spectrum, which also happens to be the part of the electromagnetic spectrum that corresponds to the Sun’s peak output. The albedo of ice on the surface of the Earth ranges from about 0.5 to 0.7; the albedo of snow is as high as 0.9. This means that icebergs and other sea ice reflect between 50% and 70% of incident visible light that hits them, and the snow that covers the ground in polar regions and that is present in high altitudes or cold areas in the winter reflects about 90% of incident light. In other words, a large fraction of the energy incident upon snow and ice is immediately reflected away, and is not absorbed, and, thus, does not contribute to raising the temperature of the Earth’s surface. Visible light also does not interact strongly with greenhouse gases the way infrared light does, so this light isn’t captured as efficiently by the atmosphere and generally ends up radiating back out into space.
It is intuitively obvious that the albedo of ice and snow is high, because they are white. We know the Sun emits white light, which is light that comprises a variety of wavelengths and spans the entire visible spectrum; something that we see as white is reflecting a wide spectrum of light back. Things that appear to be various colors do not reflect all wavelengths back but, rather, absorb some wavelengths of light and predominately reflect back light that corresponds to the colors we see. For instance, the brown and gray earth and mountaintops that lie beneath the snow and ice are absorbing more energy than the ice and snow and are only reflecting back the specific colors we see. In terms of the numbers, soil has an albedo of about 0.17, granite has an albedo as high as 0.35, and sand has an albedo of about 0.40. Other things that might lie below snow and ice, such as grass, water, trees, and other types of rocks all have albedos that are lower than ice or snow. Thus, for every small increase in temperature across the globe that results in the presence of ice diminishing on the surface of the Earth, the Earth retains more of the incident solar energy upon it and warms up. This, in turn, results in infrared radiation that can be absorbed and reemitted back to the surface of the earth by greenhouse gases in the atmosphere, causing the heat to be retained longer, making it harder for new snow and ice to form.
Temperature affects water as well as ice. Water itself has a very low albedo and absorbs a high percentage of the incident energy upon it. Water has a high heat capacity, meaning it can absorb a lot of energy without its temperature rising very much, but with so much water on the Earth’s surface, it doesn’t take very much additional energy to produce extra water vapor. Anyone who has ever washed dishes and put them in a dish rack is aware that water evaporates. The oceans, rivers, and lakes of the world are constantly giving off water vapor as some tiny fraction of their water content turns into vapor and rises into the air. This fraction rises as the temperature goes up, with all water vaporizing once the boiling point is crossed. While water on the Earth’s surface is not a major problem for global warming, water vapor is a greenhouse gas, meaning it works alongside carbon dioxide and other greenhouse gases to contribute to heat retention by the atmosphere. In fact, water vapor is the most significant contributor to the greenhouse effect in Earth’s atmosphere.
These mechanisms working together are what can cause a runaway greenhouse effect: more carbon dioxide in the atmosphere leads to a small increase in global temperatures; a small increase in global temperatures leads both to less snow and ice being present on Earth and more water vapor being present in the atmosphere; less snow and ice on Earth and more water vapor in the atmosphere leads to more incident solar energy being retained by the surface of the Earth and more heat being retained by the atmosphere; more energy in the Earth’s surface and atmosphere leads to further increases in global temperature. This starts with us, with our emission of greenhouse gases to the point that an accelerative process of heat retention by the Earth’s atmosphere leads to global temperature increases. Venus, our closest neighbor, likely had a runaway greenhouse effect at some point in the past, and its temperature finally settled at a cozy 863 °F. Venus got so hot during this process that the carbon dioxide trapped in its surface was cooked out and released into the atmosphere, which now retains energy to the point that it is as hot as it is. Earth is farther from the Sun than Venus and likely would never settle at a temperature that high (until the Sun starts to heat up and expand in a few billion years, of course…), but only a few degrees will be enough to disrupt civilization by way of global ice melt and sweeping changes to the climatological conditions in various regions, forcing mass human migration and other attempts at adaptation. I don’t particularly want to gamble on what happens after that and whether the runaway greenhouse continues to the point that the oceans boil. It will already be much too late for us by then.
I hope this serves as an illustration for why climate change happens, even if the nitty-gritty specifics of how and how much are left to the scientists. We do not need a team of scientists to tell us that a pizza that just came out of the oven is hot anymore than we need to turn to their authority to tell us whether or not global warming is real. Arguments that hinge on that are unconvincing, but, when it is so easy to understand the mechanisms of action of climate change on a fundamental level — why it works — it is unnecessary to make fallacious appeals to authority. The key elements at play are that heat is transported in the infrared wavelengths; greenhouse gases, including carbon dioxide, absorb infrared light; reemissions of photons from a molecule that has absorbed energy happen in a random direction, and the human output of carbon dioxide over time is non-negligible in comparison to the amount of carbon dioxide in the atmosphere.
